We studied the organization, match, update and applications of the design modular instance library and modular cost instance library in the environment of collaborative product design, based on modular design and with comprehensive use of the RDR and the CBR technology. We implemented the instance updating process which can read the attribute value of design parameters from the design module instance library directly, optimize the modular cost and choose the right cost modular which meets the designed target, and rewrite the new instance into the modular cost instance library automatically. A cost control decision system based on design modular instance library and cost modular instance library is designed.